The present invention relates to a curing composition, more particularly to a curing composition suitable for use in a pressure sensitive adhesive, a sealing material, and the like.
As conventional pressure sensitive adhesives there are usually used rubber adhesives obtained by using a natural rubber or a synthetic rubber as a main component and adding additives such as a tackifier, a softening agent and a filler, and acrylic resin adhesives obtained by using an acrylate as a main monomer and copolymerizing it with a polar monomer such as acrylic acid or vinyl acetate.
The preparation of the rubber adhesives, however, requires complicated procedures such as mastication of rubber chips, dissolution of the rubber and the other additives in a solvent for a long time. Also, the acrylic adhesives have insufficient cohesive force due to the problems with respect to degree of polymerization and distribution of molecular weight of an acrylic polymer. For improving cohesive force, various cross-linkable acrylic resin pressure sensitive adhesives are studied. As processes for preparing the cross-linkable acrylic resin adhesives, there are proposed a process in which a cross-linking agent such as a polyisocyanate compound, an epoxy compound, a polyamine compound, a phenol resin, a sulfur compound or an organometallic compound, and a process in which a polymerizable monomer having a hydrolyzable silicon-containing group is copolymerized with an acrylate polymer and the obtained copolymer is cross-linked with a silanol condensing catalyst. According to those processes, though the cohesive force of the acrylic resin adhesive can be improved, the preparation becomes difficult because an excess of cohesive force makes the initial thickness lower, especially at a low temperature, and the adjustment of the balance of the adhesive strength is not easy. Particularly in case of using an isocyanate compound as a cross-linking agent, it is necessary to consider not only its toxicity, but also secondary reactions.
The present inventors already proposed a curing composition containing as an effective component a specific polyether having a hydrolyzable silicon-containing group in Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 56153/1980. It is also reported that the curing composition is useful as a pressure sensitive adhesives. The curing composition has a good tackiness at a low temperature and a good cohesive force, but has an insufficient adhesive strength. Therefore, in case that peeling strength is required, the adhesive strength of the curing composition is improved by adding a relatively large amount of a tackifier being compatible with the polyether to the curing composition. In such case, however, there are a problem that application workability is lowered due to increased viscosity of the composition, a problem that when the curing composition is applied on a backing such as a plastic film, the applied composition is easy to peel off from the backing because of bad stickness of the composition to the backing, and a problem that the curing composition is inferior in weather resistance resulting from the tackifier.
Polyethers having in the molecule at least one functional group such as a reactive silicon-containing group are disclosed, for instance, in Japanese Examined Patent Publication (Tokkyo Kokoku) No. 36319/1970, No. 12154/1971 and No. 32673/1974 and Japanese Unexamined Patent Publication No. 156599/1975, No. 73561/1976, No. 6096/1979, No. 13767/1980, No. 13768/1980, No. 82123/1980, No. 123620/1980, No. 125121/1980, No. 131021/1980, No. 131022/1980, No. 135135/1980 and No. 137129/1980. Though these polyethers provide cured materials having a high elongation, these polyethers have drawbacks resulting from the structure of the main chain. For instance, the polyether proposed in Japanese Unexamined Patent Publication No. 73561/1976 is poor in weather resistance because of having urethane bonds in the main chains. In the polyether having polypropylene oxide as a main chain, when an antioxidant is not used there is a problem in weather resistance since hydrogen atoms bonding to tertiary carbon atoms are easy to be oxidized.
It is an object of the present invention to provide a curing composition having excellent initial tackiness, adhesive property, cohesive force, weather resistance, application workability and stickness to a backing, and being useful particularly as a sealing material and a pressure sensitive adhesive.
These and other objects of the present invention will become apparent from the description hereinafter.